Boucan Canot beach on Réunion, a tiny French island off the coast of Madagascar, is the kind of place where women sunbathe topless, couples picnic, men in Speedos throw Frisbees, and 80-degree water laps powder-white sand. It's postcard perfect, but for one small detail: No one at the beach can go in the water – it's against the law.

Local officials imposed a ban on swimming in October 2011, and for a very good reason: Boucan Canot, and nearby Saint-Gilles harbor, has become the shark-attack capital of the world. A tourist surfing a mile from Boucan Canot was bitten in the leg hours after arriving on the island. A week later, a shark charged a kayak, hitting it from beneath the prow and sinking it (the kayaker survived). A bodyboarder was dragged underwater and devoured just off a crowded beach. A spearfisherman wading in chest-high water was bitten in the ass. Five violent attacks occurred over nine months, two of them fatal. The worldwide average for shark fatalities is six a year – and a third of that number had occurred in tiny Réunion in less time. It was the most dramatic increase in shark attacks Réunion had ever seen, and it threatened to destroy the tourism industry that supports the island's economy.

Local marine experts believed the problem could be solved only if it could be determined why the sharks were venturing so close to the beaches. This meant finding someone with the knowledge, expertise, and courage to spend an extended period of time in the water with the animals themselves, unraveling the mystery. They called Belgian Fred Buyle, the world's foremost shark tagger, a gifted free-diver able to hold his breath for seven minutes and swim to depths below 300 feet. Buyle has been known to sit peacefully on the ocean floor surrounded by schools of man-eaters. Sharks aren't frightened by free-divers, who, unlike scuba divers, don't create anxiety-provoking bubbles. This enables Buyle to clip tiny transmitters to their dorsal fins so they can be tracked – and swimmers warned when they get too close. The transmitters would work like a missile defense system, warning Réunion officials of incoming sharks so they could order swimmers out of the water. The project, called Sharkfriendly, is a huge step toward implementing the first-ever real-time shark-tracking system.

Buyle and his fellow free-divers have become an indispensable factor in protecting humans from shark attacks – and sharks from humans. Tracking sharks means being able to alert swimmers of their whereabouts, essentially telling people where it is safe to swim. Buyle, a staunch conservationist, wanted to ensure that Réunion didn't follow the example of the neighboring Seychelles and Mauritius islands by killing off its population of bull sharks, a particularly aggressive species responsible for the attacks. He came to Réunion not only to solve the shark-attack problem but also to show that humans and bull sharks can coexist peacefully in the water.

When Buyle arrived on Réunion last November, he was welcomed at the airport by a throng of curious islanders and desperate government officials. Tall and well built, with a clean-shaven head, he carried an enormous duffel bag full of spearguns and diving equipment to a waiting car. "Sharks don't like to eat humans," he said. "That's what was so weird about these attacks. Something was frightening them, bringing them close to shore, and it was probably man-made. That's what I wanted to find out."
Three days later, Buyle is in a motorboat off the coast of La Possession marina, which is where the remains of a woman killed by a bull shark were recovered in October. With him are Fabrice Schnöller, a local marine researcher, shark expert, and free-diver; Markus Fix, a German computer programmer and the man responsible for the technical aspects of Sharkfriendly, wearing a T-shirt that reads science: it works, bitches; William Winram, a champion free-diver from Canada, who has joined Buyle on several previous tagging missions; and Guy Gazzo, a 74-year-old local free-diver and spearfisherman.

While the crew members don their gloves, boots, and wetsuits, and grab their goggles and spearguns, Buyle descends into the diamond-clear water below. He kicks a straight line down, descending 60 feet, deep into the shadows of the ocean. Finally, at the seafloor, he stops, crosses his legs, and takes a seat on a patch of white sand. He sits for five full minutes, holding his breath, watching and waiting for the sharks to come.

Buyle goes to great lengths to convince people that tagging man-eating sharks is perfectly safe. "Do I look like some adrenaline junkie?" he had said, the day before, in his rental apartment at Boucan Canot. With his soft-spoken voice, French accent, and perpetual grin, Buyle exudes a mellow calm. "Skydiving, jumping with bikes – I hate all that shit," he said. "Free-diving with sharks is the opposite of an adrenaline sport. You can't do it if you're aggressive – you need to be balanced. You need to know yourself."

Buyle showed me video footage of him swimming with tiger sharks, grabbing hold of the back of their dorsal fins, diving hundreds of feet deep into a school of hammerheads. The deeper Buyle goes beneath the surface, the more he becomes a part of the underwater environment, as if he actually belongs down there. The sharks, at least, seem to think he does. As he approaches them, the sharks go from skittish to curious, almost playful. "What people don't understand," Buyle says, "is that when you come down to their level, look them in the eye, they get it. We're just no longer on the menu."

"We're bigger than something the sharks would want to eat," says Peter Klimley, a pioneer in shark-tagging research and director of the Biotelemetry Laboratory at the University of California, Davis. "Sharks show aggressive displays when they are frightened. Buyle is very good at what he does – the guy is in control."

Growing up in an adventurous family, Buyle was diving at age seven, spearfishing by 10, and swimming with sharks at 13, while diving in Indonesia. "I saw no signs of aggression," says Buyle. "I was happy to dive with them." He discovered free-diving in his early twenties, and by 28, he had claimed four world records in the sport, once swimming downward 338 feet in a single breath. These days, Buyle spends most of his time photographing marine animals, lecturing at diving events, leading free-diving tours, and, when he's lucky, doing what he loves best: educating the public about sharks, which he considers the most misunderstood animal on the planet. "For centuries, all we heard were stories about navigators going around the world, getting eaten by sharks," he says. "The fact is, nobody knew anything about sharks. Humans fear what they don't know. When we're afraid of something, we try to get rid of it, to kill it. It's basic human behavior."

On the motorboat, as Buyle and the other divers troll for bait, Schnöller and Fix try to attract bull sharks to the area by blasting the water with the sound of wounded fish. "The recording is from 1966 – it's the only one I could find," says Schnöller, using a jury-rigged car stereo hooked up to a waterproof speaker to play the sound of a crinkling plastic bottle, which happens to sound like the scream of a crippled kingfish. Schnöller says a fisherman in Australia discovered that sharks, which have keen audio senses and can home in on prey from up to 800 feet away, were attracted to the sound of AC/DC – "You Shook Me All Night Long," in particular. "What they're listening for is random bursts of low frequencies," he says. "There's a lot of that in AC/DC." To that end, the team will try its own heavy metal shark-attractor test by blasting the water with German quasi-fascist band Rammstein – Fix's idea. "The long-haired shark will like it," says Schnöller.

Gazzo goes down next, kicking his carbon-fiber fins slowly, his speargun close at hand. The divers wear wetsuits less for insulation – the water temperature here is a balmy 78 degrees – and more to deter accidental attacks. Sharks, before they bite, often conduct a kind of taste test by bumping their noses into prey and emitting a short blast of electrical signals. If the signals conduct, as they do against animal or human flesh, there's a good chance the shark will attack. Wetsuits dull these electrical signals, reminding the sharks that, as Buyle puts it, we're not on the menu.

Buyle follows, paddling close behind. At 40 feet, he reaches neutral buoyancy, the depth at which the ocean stops pushing you to the surface and starts pulling you to the seafloor, and he puts his arms at his side and glides effortlessly down, as if being sucked to the bottom. Meanwhile, Winram is flapping his arms and legs at the surface, pretending to be a wounded seal while keeping a watchful eye on the water below.

With no sharks in sight, the divers spear a few fish and gut them inside a makeshift strainer made from a discarded washing-machine drum. The water is now full of blood and thumping with the sound of screaming kingfish. The team fixes the specially designed transmitter tags to spearguns and readies itself to go deep again. A few minutes later, Buyle pops his head from the water. "Oui, shark!" he says. He's spotted a bull shark on the ocean floor and motions to Gazzo and Winram to follow him down. Minutes pass before they resurface. Unfortunately, they've lost the shark, and there are no more to be found.

"They are just so nervous," says Buyle. The men spend another four hours diving before finally giving up. "I've never seen anything like this," he adds, shouting above the din of the motors as the team heads back to shore. "In Madagascar, Mexico, Polynesia, you dive, and there are sharks everywhere. You can't help but be around them."

Buyle believes that what is bringing the sharks to Réunion to feed so close to shore is not shark overpopulation but some sort of disturbance in the underwater environment. That they fail to discover sharks today is disappointing, but it also suggests that Buyle might be right. "The first thing we learned here," he says, "is that Réunion just doesn't have many sharks."
Réunion is rapt, following the exploits of Buyle and his team. Local newspapers run cartoons depicting the free-divers tagging sharks; reporters and radio stations call daily, asking for updates.

Two weeks into his trip, while Buyle and the team dive and bait near a fish farm not far from Boucan Canot, he finds his sharks. When they arrive in the boat, the water beneath them is swarming with bull sharks. Big ones. After a few warm-up dives, Buyle clenches his speargun and heads in with Winram. As they approach a depth of 120 feet, a shark notices Buyle and diverts its path to him. Buyle takes aim and shoots. The speartip, which is as sharp as a nail and could pass through a human head from 20 feet away, bounces off the shark's cartilage, leaving the tagging unit flaccid on the seafloor. They have found Réunion's sharks, but they will have to return another day, better equipped, in order to place the transmitters.

In the 1940s and 1950s, researchers began tagging sharks in an effort to understand their migratory routes, population, and habits. Actual plastic tags eventually gave way to transmitters in the mid-1960s. The transmitters, once adhered to a shark, could trace depth, swimming speed, and location. With the proliferation of satellite and acoustic technologies in the past 10 years (GPS technology doesn’t work under­water), shark tagging provides researchers access to intricately detailed data on shark body temperature, feeding patterns, location, and behavior. The latest tagging technology is an acoustic-based system using cigar-size transmitters affixed to the shark's dorsal fin. The beacons send out a signal via 10 relay hubs. When the shark approaches within 500 meters, the hub recognizes the acoustic beacon like a fingerprint and sends an instant alert to a computer server, which updates a website and mobile app.

Today, from Western Australia to South Africa and Florida, thousands of sharks glide through the world's oceans, transmitting their every move to satellites. But the challenge for researchers hasn't been developing the technology but getting it onto the sharks in the first place. Tagging from boats can put the sharks at risk – wayward spears can kill them. Tagging from cages or with scuba doesn't work because the sharks become nervous and swim away or approach at the wrong angle. Free-divers, who are unobtrusive and can swim hundreds of feet below the surface without disturbing the sharks, make optimal shark taggers.

Buyle's first job tagging sharks was on the island of Malpelo, off the west coast of Colombia, in 2005. Researchers had speculated that hammerhead sharks in the area could be migrating as far south as the Galápagos Islands, some 1,400 miles away. If they were, Colombia could establish the whole region as a marine reserve, protecting the shark. But first they'd need to prove it. Over three trips, Buyle dove to depths of more than 200 feet and tagged 150 hammerheads. The data that came back was startling. Not only were the hammerheads migrating to waters around the Galápagos, and farther away, but they were doing so in packs of several hundred, swimming head to fin with one another like stealth fighters. As a result, in 2006, 3,300 square miles around Malpelo was named a Unesco World Heritage site.
Buyle, who had volunteered to come to Réunion for three weeks, soon returned home but not without first retooling the spearguns to shoot at double strength. Three days after he departed, Gazzo successfully tagged three bull sharks, enough for an initial test of the tracking system. Schnöller and Gazzo then spent the next month watching tagging data, trying to identify patterns. The bull sharks congregated around Boucan Canot and its nearby harbor, so they returned to the area to investigate, free-diving along the seafloor outside Saint-Gilles marina. What they discovered was an enormous trash heap of plates, food, and refuse that boaters had been throwing overboard at the port entrance north of the beach. The bull sharks gathered off Boucan Canot to feast, and it could be that the humans they attacked were getting in the way.

"We accomplished our goal," Schnöller said, a week later. "We are educating people about the problem, showing why they can't just throw trash in the ocean." Schnöller is also using the discovery as a way to inform local beachgoers and tourists about shark habits, which will hopefully prevent future attacks.

Buyle, in Brussels, agrees it's a useful discovery, but he also says it may have little effect on the number of shark-related incidents on Réunion. "We will never change the shark's behavior," says Buyle. The most important thing is to "respect that the ocean is a wild environment. This is the shark's home. You are just a visitor in it."

Surprisingly, the tagging discovery didn't scare people off but, instead, opened a new cottage industry. Today Réunion tour operators run snorkeling trips near the garbage dump. "The sharks have become an attraction," said Schnöller. Last January, the French government began reopening beaches to the public.

I asked Buyle if he now considers Boucan Canot a safe place to swim or surf. "I'll put it this way," he said, laughing. "Without my wetsuit, goggles, and fins, I wouldn't swim in that water if you paid me a million dollars. I'm not crazy, you know."